Seismic Performance of Square Steel Tube Recycled Concrete Columns

Literature Overview

The study by Li Bing, Meng Shuang, and Ji Fengying (2017), published in the Journal of Guangxi University (Natural Science Edition), investigates the seismic behavior of square steel tube recycled concrete (RC) columns using nonlinear finite element analysis in ABAQUS. Funded by the National Natural Science Foundation of China (Grant No. 51578347), this research addresses a growing concern in sustainable construction: how the substitution of recycled coarse aggregate (RCA) for natural coarse aggregate affects the seismic capacity of composite columns. The authors model the interface between the steel tube and core concrete using a Coulomb friction model and apply combined horizontal and vertical loading to simulate realistic seismic conditions. The key finding is that seismic performance degrades with increasing RCA replacement rate, and a simplified skeleton curve formula is proposed for practical design use.

Core Technical Findings

The nonlinear finite element model employs a one-end-fixed, one-end-fixed-rotation boundary condition to replicate actual column behavior in a structural system. The Coulomb friction model at the steel-concrete interface is critical because it governs the confinement mechanism that distinguishes composite columns from conventional reinforced concrete columns. The hysteresis curves of the square steel tube recycled concrete columns are described as relatively full and capable of good energy dissipation, which is consistent with experimental observations from existing literature.

The authors derive a simplified skeleton curve for square steel tube recycled concrete columns by referencing the characteristic values of conventional square steel tube concrete columns. The general formula adjusts the peak strength and post-peak degradation parameters based on the RCA replacement rate. This provides a practical tool for engineers who need to estimate seismic demand without performing full nonlinear analysis for every design iteration.

Interpretation of Technical Points

The degradation of seismic performance with increasing RCA replacement rate can be attributed to several material-level mechanisms. Recycled coarse aggregate typically exhibits higher water absorption, lower specific gravity, and a rougher surface with adhered old mortar compared to natural aggregate. These characteristics lead to a weaker interfacial transition zone (ITZ) between the aggregate and the cement paste, which in turn reduces the compressive strength and elastic modulus of the recycled concrete. In a composite column under cyclic loading, the core concrete is confined by the steel tube, but the lower stiffness and strength of recycled concrete means that the confinement benefit is partially offset. The steel tube still provides significant lateral restraint, which is why the hysteresis loops remain relatively full even at high replacement rates, but the overall displacement capacity and energy dissipation diminish.

From a welding and fabrication perspective, the square steel tube used in these columns is typically manufactured by cold-rolled or hot-finished steel strips joined by longitudinal and transverse welds. The quality of these welds is paramount because they are potential initiation sites for local buckling under cyclic compression. Engineers should ensure that the welds meet the acceptance criteria of GB/T 3323 or ISO 5817 for radiographic testing, and that the weld toes are ground or chamfered to reduce stress concentration. The steel grade is commonly Q345 or Q355 according to GB/T 1591, and the tube dimensions should comply with GB/T 6728 for cold-formed square and rectangular hollow sections.

Engineering Practice Considerations

In practical seismic design, the skeleton curve derived by the authors can be incorporated into pushover analysis or equivalent lateral force procedures. However, engineers must be cautious about extrapolating beyond the tested replacement rate range. The study's conclusions suggest that when the RCA replacement rate is reduced, the component exhibits better seismic capacity. This implies that for high-seismicity regions, the RCA replacement rate should be limited, possibly to below 50 percent, to ensure adequate safety margins.

The Coulomb friction model used at the interface is a simplification. In reality, the bond between the steel tube and concrete involves chemical adhesion, mechanical interlock, and friction, all of which evolve under cyclic loading. Future research should consider more sophisticated interface models, such as the Mortar Cover model or a user-defined contact law that captures the progressive degradation of bond strength. Additionally, the study does not address the effect of steel tube local buckling on the seismic response, which is a well-known concern for slender composite columns.

Study Insights and Implications

This research contributes valuable data to the field of sustainable structural engineering by quantifying the seismic penalty associated with recycled aggregate use in composite columns. The proposed skeleton curve formula offers a practical design shortcut, but its validity depends on the accuracy of the underlying material models and the representativeness of the tested specimens. Engineers working on green building projects should use these findings as a starting point for design, supplemented by component-level testing or detailed finite element analysis for critical applications. The overarching message is that recycled concrete can be used in composite columns with acceptable seismic performance, provided that the replacement rate is carefully controlled and the design accounts for the reduced material properties.